Bi-directional HEMT/GaN half-bridge circuit

ABSTRACT

A half-bridge circuit in accordance with an embodiment of the present application includes an input voltage terminal operable to receive an input voltage, a first bi-directional switch, a second bi-directional switch connected in series with the first bi-directional switch, wherein the first and second bi-directional switches are connected to the input voltage terminal such that the input voltage is provided across the first and second bi-directional switches and a controller operable to turn the first and second bi-directional switches ON and OFF such that a desired voltage is provided at an midpoint node positioned between the first bi-directional switch and the second bi-directional switch. The first bi-directional switch and the second bi-directional switch are high electron mobility transistors structured to allow for conduction in two directions when ON and to prevent conduction in any direction when OFF.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims benefit of and priority to U.S.Provisional Patent Application Ser. No. 60/916,913 for BI-DIRECTIONALHEMT/GaN HALF-BRIDGE AND SINGLE STAGE RESONANT AND PFC CIRCUIT filed May9, 2007, the entire contents of which are hereby incorporated byreference herein.

The present application is also related to concurrently filed U.S.application Ser. No. 12/118,320 filed May 9, 2008 entitled CLASS DAMPLIFIER WITH BI-DIRECTIONAL POWER SWITCH which claims benefit of andPriority to U.S. Provisional Patent Application Ser. No. 60/941,395 forCLASS D AMPLIFIER WITH BI-DIRECTIONAL POWER SWITCH filed Jun. 1, 2007,the entire contents of both of which are hereby incorporated byreference herein.

BACKGROUND

1. Field of the Disclosure

The present invention relates to a half-bridge circuit utilizingbi-directional HEMT/GaN transistors.

2. Related Art

The traditional half-bridge circuit, sometimes known as an inverter, isone of the most fundamental circuit switch configurations used in powerelectronics today. Half-bridges are commonly used in a wide variety ofapplications, including electronic ballasts for lighting, class-D audioamplifiers, resonant mode power supplies and motor drive circuits. Thetypical half-bridge circuit 10 is illustrated in FIG. 1 and includes twoswitches M1, M2 connected in a totem poll configuration between a DC busand ground. The switches are typically MOSFETsS or IGBTs.

The drain of the upper (high side) MOSFET M1 in FIG. 1 is connected tothe DC bus voltage which is generally several hundreds of volts. If M1were an IGBT, the collector would be connected to the DC bus voltage.The source of the second (low side) MOSFET M2 in FIG. 1 is connected toground. Two free flowing diodes D1 and D2 are placed in parallel withthe switches M1, M2, respectively, with the cathode connected to thedrain and the anode connected to the source. In this manner, thehalf-bridge is formed and converts the DC bus voltage into a square-wavevoltage. A half-bridge control circuit 12 controls the on-off times ofeach of the switches M1, M2. The circuit 12 provides the correct timingof the voltages at the gates of the switches M1, M2 to set a desiredfrequency, duty cycle and dead time for the square wave. The mid-pointnode VS of the half-bridge is typically connected to a resonant typeload circuit (L1, RLOAD, C1, C2). A snubber capacitor CSNUB ispreferably connected between node VS and ground, or between VS and theDC bus, to control the dv/dt of the rising and falling edges of thesquare wave. The diodes D1, D2 provide a current path during the deadtime when neither switch is active to maintain a continuous currentthrough the load.

When M1 turns ON, current flows from the DC bus, through M1 and throughthe load circuit to ground. When M1 turns off, current continues to flowout of the snubber capacitor CSNUB through the load to ground. Ascurrent flows out of the capacitor CSNUB, the voltage at node VSdischarges linearly to ground at a rate provided by the followingequation:dv/dt=I*CSNUB.When the voltage at VS reaches 0.7 V below ground, the lower diode D2will become forward biased and current will continue to flow from groundthrough diode D2 and through the load back to ground. At some pointafter the voltage at VS reaches ground, the dead time will end and theswitch M2 will turn ON. The load current will transition from diode D2to the channel of switch M2. When the load current changes direction dueto the resonant nature of the circuit, the current will continue to flowfrom the load through the switch M2 and to ground. When M2 is turnedOFF, the load current will flow into CSNUB and charge the node VS uplinearly at a rate defined by the equation:dv/dt=I*CSNUB.When the voltage at node VS reaches 0.7 volts above the DC bus voltage,the diode D1 will become forward biased and current will flow fromground, through the load, through D1 to the DC bus and throughcapacitors C1, C2 and back to ground. When the dead time is over, M1will turn on and the cycle repeats. Thus, using the MOSFETs and thediodes D1, D2 allows for smooth and continuous current in both positiveand negative directions.

When the half-bridge 10 is connected to an AC line voltage source 14,which serves and an input voltage, as illustrated in FIG. 2, forexample, it is necessary to provide a bridge rectifier 4 to allow forbi-directional current flow from the AC line input voltage. FIG. 3Aillustrates the AC line voltage. FIG. 3B illustrates the rectifiedvoltage provided by the rectifier bridge 4. FIG. 3C illustrates thevoltage at the node VS.

The rectifier bridge 4 keeps the voltage at the drain of the high sideMOSFET M1 (or IGBT) always equal to, or above ground. This is importantsince if the DC bus voltage goes negative, the diodes D1 and D2 will beforward biased at the same time and a short circuit will result. Theshort circuit will result in the square wave becoming non-functionalwhile the high current damages circuit components. Utilizing the bridge4, however, the voltage at node VS will transition between the DC busvoltage and ground continuously at a certain frequency and duty cycle,as is illustrated in FIG. 3C. The amplitude of the square wave is givenby the DC bus voltage level. If the capacitors C1, C2 are small, theamplitude follows the peak rectified voltage as shown in FIG. 3C.

Where MOSFETs are used, the channel allows for current to flowbi-directionally, however, the diodes D1 and D2 are inherent to thedevices and cannot be removed. If IGBT switches are used, the switchesconduct in only one direction, from the collector to the emitter, andthus, diodes must be added to provide bi-directional current flow.

Accordingly, it would be beneficial to provide a half-bridge circuitthat allows bi-directional current flow without the need for theadditional diodes described above.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a half-bridgecircuit using bi-directional HEMT devices that allow for bi-directionalcurrent flow without the need for additional diodes.

A half-bridge circuit in accordance with an embodiment of the presentapplication includes an input voltage terminal operable to receive aninput voltage, a first bi-directional switch, a second bi-directionalswitch connected in series with the first bi-directional switch, whereinthe first and second bi-directional switches are connected to the inputvoltage terminal such that the input voltage is provided across thefirst and second bi-directional switches and a controller operable toturn the first and second bi-directional switches ON and OFF such that adesired voltage is provided at an midpoint node positioned between thefirst bi-directional switch and the second bi-directional switch. Thefirst bi-directional switch and the second bi-directional switch arehigh electron mobility transistors structured to allow for conduction intwo directions when ON and to prevent conduction in any direction whenOFF.

Other features and advantages of the present invention will becomeapparent from the following description of the invention which refers tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 is an illustration of a conventional half-bridge circuit;

FIG. 2 is an illustration of the half-bridge circuit of FIG. 1 connectedto a full bridge rectifier;

FIG. 3A is an illustration of the input AC voltage provided in thecircuit of FIG. 2;

FIG. 3B is an illustration of a rectified waveform provided by thebridge rectifier of FIG. 2;

FIG. 3C is an illustration of the voltage at node VS of the circuit ofFIG. 2;

FIG. 4 is an illustration of a half-bridge circuit in accordance with anembodiment of the present application;

FIG. 5A is a graph illustrating the waveform of the input AC linevoltage in the half-bridge circuit of FIG. 4;

FIG. 5B is a graph illustrating the waveform of the voltage at the nodeVS in the half-bridge circuit of FIG. 4;

FIG. 5C is a graph illustrating the waveform of the inductor and loadcurrent in the half-bridge circuit of FIG. 4;

FIG. 6 is a graph illustrating the AC line input voltage and the AC lineinput current in the circuit of FIG. 4;

FIG. 7 is a graph illustrating the waveform of the voltage at node VSand the inductor and load current when the voltage at the positiveterminal of the AC line input voltage is positive relative to that ofthe negative terminal in the circuit of FIG. 4;

FIG. 8 is a graph illustrating the waveform of the voltage at node VSand the inductor and load current when the voltage at the positiveterminal of the AC line input voltage is negative relative to that ofthe negative terminal in the circuit of FIG. 4;

FIG. 9A is a graph illustrating the waveform of the voltage at node VSin the circuit of FIG. 4;

FIG. 9B is a graph illustrating the waveform of the inductor and loadcurrent in the circuit of FIG. 4;

FIG. 9C is a graph illustrating the waveform of the capacitor current inthe circuit of FIG. 4;

FIG. 10 is an illustration of a half-bridge circuit in accordance withanother embodiment of the present application;

FIG. 11 is an illustration of a conventional Class D amplifier circuit;

FIG. 12A illustrates characteristics of a conventional MOSFET switch;

FIG. 12B illustrates characteristics of a bi-directional switch for usein the Class D amplifier circuit of the present application;

FIG. 13 is an illustration of a Class D amplifier circuit in accordancewith an embodiment of the present application;

FIG. 14 is an illustration of a Class D amplifier circuit in accordancewith another embodiment of the present application;

FIG. 15 is an illustration of a Class D amplifier circuit in accordancewith another embodiment of the present application; and

FIG. 16 is a more detailed view of a power switch selector for use inthe circuit of FIG. 14.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The half-bridge circuit 100, illustrated in FIG. 4 of the presentapplication, uses high electron mobility transistors (HEMTs) asswitches. HEMTs are preferably made using Galium Nitride and provide forbi-directional current flow without the need for the diodes D1, D2discussed above with reference to FIG. 1. Two GaN HEMT devices, orswitches (S1 and S2) are positioned in a totem pole configuration torealize a bi-directional half-bridge 100. The top and bottom switchesS1, S2 of the half-bridge 100 are connected directly to the AC lineinput voltage source 104 after the EMI filter (inductor L1). Thus, thereis no need for a bridge rectifier as in the circuit of FIG. 1. Thecircuit 100 also includes a control circuit 105 to control the timing ofthe half-bridge, a snubber capacitor CSNUB, connected from the node VSto the negative input terminal (−) of the AC line voltage (oralternatively between the node VS and the positive input terminal (+) ofthe AC line voltage). A series inductor L1 and parallel resistor RLOADand capacitor C3 form a resonant load circuit. The capacitors C1 and C2are connected in series between the positive (+) and negative (−) inputterminals of the AC line voltage. The resonant circuit (RLOAD, C3) isconnected between the VS node and the midpoint of the capacitors C1, C2.

The peak amplitude of the waveform at the node VS directly follows thepositive and negative peak amplitude of the AC line voltage as can beseen in FIG. 4B. When the (+) terminal of the AC line voltage ispositive with respect to the negative (−) terminal of the line voltage(see FIG. 5A), the VS node is positive and continuously switchingbetween the positive and negative voltage values at a given frequency,duty cycle and dead time. When the (+) terminal of the AC line voltageis negative with respect to the (−) terminal of the AC line, the voltageat the node VS is negative and is continuously switching between thenegative and positive input values at a given frequency, duty cycle anddead time. During the positive and negative peak amplitude swings of theAC line input voltage, the half-bridge peak amplitude goes positive andnegative as well, but the inductor current peak amplitude through L1(see FIG. 5C) swings positive and negative equally above and below zeroas it oscillates during high-frequency switching cycle of thehalf-bridge 100.

When the inductor current through L1 is smoothed with the inductor LINof the EMI filter, the result is an AC input line current 52 that issinusoidal and almost in phase with the AC input line voltage 50 asillustrated in FIG. 6. The AC half-bridge driven series L1, parallelRLOAD and C3 circuit automatically produces a high power factor and verylow total harmonic distortion (THD) in a single stage. Traditionalsolutions require a separate PFC converter at the front of the circuitto actively correct power factor and actively shape the current to matchthe sinusoidal shape of the AC line voltage.

During the time when the voltage at the (+) terminal of the AC line ispositive with respect to the voltage at the (−) terminal thereof, thebi-directional inductor current flow through L1 is maintained. When theswitch S1 is turned ON, switch S2 is OFF and the VS node of thehalf-bridge is connected to the (+) terminal of the AC line voltage. Theinductor current, through L1, flows from the (+) terminal through theswitch S1, through the inductor and the load. Then half of the currentflows through the upper capacitor C1 and half through the lowercapacitor C2. When the switch S1 is turned OFF, the dead-time periodbegins and the inductor current through L1 flows in the snubbercapacitor CSNUB, through the inductor L1 and then splits equally betweenC1 and C2. The voltage at node VS ramps down linearly to zero at a rategiven by:dv/dt=I*CSNUBWhen the VS node reaches the (−) terminal of the AC line, the dead timeperiod ends, the lower switch S2 turns ON and the node VS of thehalf-bride is connected to the (−) terminal of the AC line. The inductorcurrent of L1 no longer flows through CSNUB and now flows through switchS2, through the inductor L1 and then splits equally through C1 and C2.When switch S2 turns OFF, the dead time period begins again and thevoltage at node VS ramps up linearly at the following rate:dv/dt=I*CSNUBWhen the voltage at node VS reaches the voltage at the (+) terminal ofthe AC line, switch S1 turns ON again and the cycle repeats. The VS nodecontinues to transition back and forth between the (+) terminal voltageand zero at the given frequency, duty-cycle and dead time. FIG. 7illustrates the waveform of the voltage at the node VS as trace 70 whichswitches between 0 and the positive peak amplitude of the AC linevoltage. Trace 71 illustrates the peak positive AC line voltage whilethe trace 72 illustrates the continuous inductor and load voltage.

FIG. 8 illustrates the same waveforms as FIG. 7 during the time when the(+) terminal of the AC line voltage is negative with respect to the (−)terminal of the AC line voltage. The inductor bi-directional currentflow is also maintained as is illustrated by trace 82. The timing of theswitches S1, S2 remains the same, and the only difference now is thatnow the (+) terminal of the AC line is negative with respect to the (−)terminal so that the voltage at the VS node voltage indicated by trace80 transitions between zero and a negative peak amplitude of the AC linevoltage (trace 81) at the given frequency, duty-cycle and dead time.

In a preferred embodiment, the half-bridge circuit 100 also includeszero-voltage crossing (ZVS) circuit 200 (see FIG. 4) which is preferablyimplemented in the control circuit 102. The zero voltage crossingcircuit 200 may be implemented in any desired manner, but is preferablyintegrated into controller 102. The ZVS circuit 200 is useful because nofree flowing diodes can be used due to the direct connection between theAC input line voltage and the half-bridge circuit 100. The ZVS circuit200 is active during the dead time to detect exactly when the voltage atthe VS node has slowed completely to the (−) terminal line connectionbefore turning on the low-side switch S2 and conversely detects exactlywhen the VS node has slowed completely to the (+) terminal line inputbefore turning on the high side switch S1. By precisely controlling theturn on of each switch at zero volts, the load current will transitiondirectly from the snubber capacitor CSNUB to the switch S1 or S2 withoutthe need for the free flowing diodes. In FIG. 9A, the waveform of thevoltage at node VS is illustrated. FIG. 9B illustrates the inductor andload current while FIG. 9C illustrates the snubber capacitor current ofCSNUB. The ZVS circuit 200 ensures that continuous current is maintainedto the inductor and load as is illustrated.

FIG. 10 illustrates a complete circuit for producing an isolated outputvoltage using the half-bridge circuit 100 discussed above. That is, theoutput voltage VOUT provided to the load in FIG. 10 is isolated from thehigh voltage AC input line voltage. A transformer T1 and bridgerectifier BR1 are provided between the node VS and output circuit(CLOAD, RLOAD). The transformer secondary to primary winding ratiodetermines the step up or step down ratio and therefore determines thedesired output voltage VOUT. In contemporary isolated circuits, a lowfrequency AC line voltage is converted to DC via the rectifier at theinput then converted back to high frequency AC through the half-bridgeand then to the final DC output through the bridge rectifier andsmoothing capacitor at the output. In the circuit of FIG. 10, a lowfrequency AC voltage is input and converted to a high frequency ACvoltage at the half-bridge 100, to provide an isolated output voltageVOUT to the load RLOAD through the bridge rectifier BR1 and smoothingcapacitor COUT. The AC to DC step is removed at the input and high powerfactor is inherently provided. The circuit of FIG. 10 operates insubstantially the same manner as that of FIG. 4 except that thetransformer T1 and bridge BR1 provide for isolation between the highvoltage AC line voltage and the load.

The half-bridge circuit of the present application may be used in avariety of applications that require a half-bridge circuit. Thehalf-half-bridge circuit of the present application however allows fordirect connection to an AC line voltage and thus eliminates the need fora DC bus and rectifier.

One exemplary and non-limiting application for half-bridge circuit is inClass D amplifiers. FIG. 11 illustrates an example of a conventionalClass D amplifier. In a conventional power amplifier, there aretypically two stages, a DC/DC converter stage 301 that provides adesired bus voltage and the Class D amplifier stage 302. Each stage isbuilt based on one of the switching power supply topologies. In thesesystems, the bus capacitors 5 at the output of the converter stage 301consume considerable space. The losses in a system such as thatillustrated in FIG. 11 are primarily from four items, the primary switchor switches 2, the transformer 3, the secondary rectifier 4 and theClass D amplifier switch, or switches 6, 7.

Generally, the control circuit 1 controls the primary switched 2 toprovide a desired voltage to the primary coils of the transformed 3. Thesecondary coils of the transformer 3 are connected to the rectifier 4which rectifies the voltage induced in the secondary coils by the inputvoltage at the primary coils. This rectified voltage is used to providea DC bus voltage across the bus capacitors 5. This DC bus voltage is inturn provided to the Class D amplifier stage 302.

In the circuit of FIG. 11, in order to obtain higher and/or bipolar busvoltages, the switched mode power supply converts input voltage into abus voltage for the Class D amplifier. Since Class D amplifiers requirea DC bus voltage, the secondary output of the switched mode power supplyis fed to rectifier 4 and charges the bus capacitors 5. The Class Damplifier typically uses power MOSFETs as the switches 6, 7. As is notedabove, MOSFETs have intrinsic body diodes. These body diodes limitconduction area to three quadrants as is illustrated in FIG. 12A.

A Class D amplifier circuit 400 in accordance with the presentapplication utilizes bi-directional power switching devices such as theHEMT GaN devices described above in place of the MOSFETs describedabove. An HEMT GaN device allows conduction in all four quadrants, as isillustrated in FIG. 12B, for example. Utilizing these switches in theClass D amplifier stage 402 allows the use of an AC input voltage.Further, using these bi-directional switches, output PWM is provideddirectly from bipolar power supply output pulses. The Class D amplifierof the present application thus eliminates the need for the lossysecondary rectifier 4, illustrated in FIG. 11, for example. In addition,the bus capacitor 5 can also be eliminated since the DC bus is no longernecessary.

FIG. 13 illustrates the Class D amplifier circuit 400 in accordance withan embodiment of the present application. While this circuit operates ina manner similar to that of FIG. 11, the MOSFETS 6, 7 of FIG. 1 arereplaced by the bi-directional power switches 10 and 11. These switchesare preferably HEMT GaN type switches, similar to those utilized in thehalf-bridge of FIG. 4, for example. In addition, the Class D amplifiercircuit 400 of the present application eliminates the need for thesecondary rectifier 4 and the bus capacitors 5 since there is no longera need for a DC bus at all. This makes the circuit significantly smallerand more efficient.

In general, the controller 1 controls the switches 2 to provide adesired voltage to the primary of the transformer 3. However, thesecondary of the transformer 3 may be connected directly to the Class Damplifier stage 402, without the need for rectification. The converterstage 401 is substantially minimized in components and in space whencompared to the circuit of FIG. 11.

In addition, error correction in the Class D amplifier stage 402eliminates power supply dependency gain and removes the need for aregulated power supply stage, which also improves efficiency. The ClassD amplifier circuit 400 uses a switched mode power supply with a bipolaroutput voltage such as a push-pull, half-bridge or full bridge. Thesedevices may have high utilization of core magnetization in order toprovide a smaller size.

The Class D amplifier circuit 400 of FIG. 13 represent a PWM converterthat utilizes bi-directional power switches. There is not need forsecondary rectification or for bus capacitors. Further, the PWMconverter has an AC input and an AC output, where the frequency of theoutput is preferably lower than that of the input.

FIG. 14 illustrates an alternative embodiment of a Class D amplifiercircuit 500 in accordance with an embodiment of the present application.In this embodiment an auto-transformer based push-pull primary isutilized in the converter stage 501. The primary controller 22 controlsthe power switches 20 and 21 to provide the desired input voltage viathe push-pull primary. PWM control is provided via the PWM controller 23and the selector 24. Since the polarity of the input voltage may change,the selector 24 is used to select the appropriate PWM control signal tobe provided to the switches 10, 11 based on the polarity indicationsignal 25. FIG. 16 illustrates a more detailed illustration of the powerswitch selector 24. As illustrated, the selector 24 uses logic gates160, 162 and takes as inputs the PWM signal and the polarity indicationsignal 25 to provide the HO and LO signals to drive the switches 10 and11 such that they are not ON at the same time.

FIG. 15 illustrates another embodiment of a Class D amplifier circuit600 in accordance with an embodiment of the present application whereinthe primary of the transformer 3 is provide with an input voltage via afull bridge formed by the switches 2. Otherwise this circuit operates insubstantially the same manner as that of FIG. 13.

Thus, the Class D amplifier circuit of the present application utilizesbi-directional power switches without internal diodes in order toeliminate the need for the secondary rectifier, DC bus and buscapacitors which saves space and reduces losses. The bi-directionalpower switches are preferably HEMT GaN type transistors of the typedescribed herein. In this manner, the input voltage to the amplifier maybe an AC input voltage.

Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art. It ispreferred, therefore, that the present invention be limited not by thespecific disclosure herein, but only by the appended claims.

1. A half-bridge circuit comprising: an input voltage terminal operableto receive an input voltage; a first bi-directional switch; a secondbi-directional switch connected in series with the first bi-directionalswitch, wherein the first and second bi-directional switches areconnected to the input voltage terminal such that the input voltage isprovided across the first and second bi-directional switches; and acontroller operable to turn the first and second bi-directional switchesON and OFF such that a desired voltage is provided at an midpoint nodepositioned between the first bi-directional switch and the secondbi-directional switch, wherein the first bi-directional switch and thesecond bi-directional switch are high electron mobility transistorsstructured to allow for conduction in two directions when ON and toprevent conduction in any direction when OFF.
 2. The half-bridge circuitof claim 1, wherein the first bi-directional switch and the secondbi-directional switch are gallium nitride type transistors.
 3. Thehalf-bridge circuit of claim 2, wherein the input voltage terminalfurther comprises a positive terminal and a negative terminal and theinput voltage is an AC voltage, and wherein the first and secondbi-directional switched are connected between the positive terminal andthe negative terminal of the input voltage terminal.
 4. The half-bridgecircuit of claim 3, wherein the first and second bi-directional switchesare structured such that they do not include internal diodes.
 5. Thehalf-bridge circuit of claim 4, wherein the midpoint node is connectedto a load and the desired voltage is provided to the load based on theON/OFF status of the first and second bi-directional switches.
 6. Thehalf-bridge circuit of claim 5, further comprising: a transformerconnected between the midpoint node and the load; and a rectifier bridgeconnected to a secondary coil of the transformer and connected to theload such that the load is magnetically isolated from the midpoint node.7. The half-bridge of claim 6, wherein a primary coil of the transformeris connected to the midpoint node and the secondary coil is connected tothe rectifier bridge such that a DC output voltage is provided to theload.
 8. The half-bridge of claim 5, further comprising a zero crossingcircuit operable to determine when the voltage at the midpoint nodecrosses zero.
 9. The half-bridge of claim 8, wherein the zero crossingcircuit is positioned in the controller and provides a zero detectsignal indicating when the voltage at the midpoint node crosses zero.10. The half-bridge of claim 9, wherein the first and secondbi-directional switches are turned ON by the controller based on thezero detect signal provided from the zero crossing circuit.
 11. Ahalf-bridge circuit comprising: an input voltage terminal operable toreceive an input voltage; first and second bi-directional high electronmobility transistors (HEMTs) connected in a totem pole configurationdirectly across said input voltage terminal; a control circuit operableto turn said first and second bi-directional HEMTs ON and OFFindividually such that a desired voltage is provided at a nodepositioned between said first and second bi-directional HEMTs, andwherein said first and second bi-directional HEMTs are structured toallow for conduction in two directions when ON and to prevent conductionin any direction when OFF without external diodes connected across theirterminals.
 12. The half-bridge circuit of claim 11, wherein said firstand second bi-directional HEMTs comprise III-nitride devices.
 13. Thehalf-bridge circuit of claim 11, wherein said first and secondbi-directional HEMTs are comprised of gallium nitride (GaN).
 14. Thehalf-bridge circuit of claim 11, wherein the input voltage is an ACvoltage and said first and second bi-directional HEMTs are connectedbetween a positive terminal and a negative terminal of said inputvoltage terminal.
 15. The half-bridge circuit of claim 11, wherein thefirst and second bi-directional HEMTs are structured such that they donot include internal diodes.
 16. The half-bridge circuit of claim 15,further comprising: a transformer connected between said node and aload; and a rectifier bridge connected to a secondary coil of saidtransformer and connected to said load such that said load ismagnetically isolated from said node.
 17. The half-bridge of claim 16,wherein a primary coil of said transformer is connected to said node andsaid secondary coil is connected to said rectifier bridge such that a DCoutput voltage is provided to said load.
 18. The half-bridge of claim11, further comprising a zero crossing circuit operable to determinewhen a voltage at said node crosses zero.
 19. The half-bridge of claim18, wherein said zero crossing circuit is included in said controlcircuit and provides a zero detect signal indicating when said voltagecrosses zero.
 20. The half-bridge of claim 19, wherein said first andsecond bi-directional HEMTs are turned ON by said control circuit basedon said zero detect signal.